If it's a real leaded window with tiny panes of glass, held together with real leaded strips then I believe you need to use 100% pewter lead for filling in any very small gaps in the leaded strips. It can just about be annealed (worked/moulded) at hand temperature, or better still, heat it up in a kettle if need be. But then again if it's real leaded windows , why would you WFP it, because they leak like mad, lol